Use a rigid ruler or measuring tape
Measure when the penis is fully erect
Place the ruler at the pubic bone on the top side of the penis
Press the ruler gently into the pubic fat pad until it touches the bone
Measure from the pubic bone to the tip of the glans
Do not include the foreskin beyond the tip
For girth, wrap a soft measuring tape around the thickest part of the shaft
Record the measurement in inches or centimeters
Measure a few times for consistency